1. Field of the Invention
The present invention pertains to time interval to digital converters particularly with respect to smoothing the digital output to eliminate jitter.
2. Description of the Prior Art
It is often desirable in the prior art to convert a time interval to a digital signal. In practical systems in which such a conversion may be utilized, time jitter at the boundaries of the time interval may cause the digital signal to vary erratically resulting in anomalous behaviour in the system. Such time intervals are often represented by pulse width modulated signals where the width of the individual pulses are the time intervals to be converted.
Such conversion of pulse width modulated signals into digital format is encountered in airborne radar systems having an antenna mounted in a radome on the aircraft wherein the antenna scans in a reciprocating sector scan manner. A resolver coupled to the antenna shaft provides AC voltages proportional to the sine and cosine of the azimuth angle of the antenna. These sine and cosine voltages are transmitted to a display unit in the aircraft via shielded wiring. In a well-known manner the sine and cosine signals are converted to a variable width pulse where the pulse width is related to the antenna azimuth angle. The variable width pulse is converted to a digital word by known techniques to address an XY memory utilized to store the radial lines of the received radar information. Each location in the memory corresponds to an incremental azimuth angle. In a typical system, the sine and cosine voltages may be converted into a 10 bit parallel digital azimuth address word which would provide the capability of storing 1.024 radial lines of radar data in the memory. The memory is rapidly read out to drive a CRT display on which the radar data is written in PPI sector scanning manner.
Errors such as noise, hum and mechanical cogging of the antenna result in jitter in the digital addressing word. This jitter disturbs the uniform memory accessing such that radial lines of memory may randomly not be written to. This results in anomalous and undesirable random black radial lines in the CRT display giving the appearance of uneven motion of the antenna. Such uneven motion would result in no data being written to memory from the incremental azimuth angles represented on the lines.
Various techniques have been utilized in the prior art in an attempt to obviate the anomalies caused by jitter. Analog low pass filters to process the sine and cosine signals so as to filter out the jitter signals results in undesirable follow-up delay. Digital signaling techniques may be utilized to convert the antenna azimuth angle into digital format at the antenna. This requires the addition of a significant amount of circuitry to be installed in the hostile environment of the radome. A further technique utilized in the prior art is to slew a counter with a voltage controlled oscillator, the frequency of which being determined by azimuth feedback from the antenna. For example, sine and cosine potentiometers or synchros coupled to the azimuth axis of the antenna may be utilized to provide these signals. Such a technique suffers from the disadvantage that directional reversals of the sector scanning antenna cannot be accurately followed at the end points. Thus, the prior art techniques cannot provide an accurate digital representation of the position at the end points when the antenna is experiencing a reversal from full scanning speed in one direction to full scanning speed in the opposite direction.